Generally, in an air-conditioning apparatus in which an indoor heat exchanger and an outdoor heat exchanger are connected to a compressor via coolant piping, when the compressor stops operating, the coolant has a tendency to move toward the coolest portion in the refrigeration cycle and to get condensed.
For example, when the operation of an air heater is stopped during nighttime, the coolant in the indoor heat exchanger moves to the compressor or to the outdoor heat exchanger, because the compressor or the outdoor heat exchanger are at a lower temperature as compared to the indoor heat exchanger. As the outside air temperature rises in the morning, the outdoor heat exchanger undergoes a faster rise in temperature because of its smaller heat capacity than the heat capacity of the compressor. Hence, the coolant inside the outdoor heat exchanger moves toward the compressor having a lower temperature and gets condensed. That leads to the occurrence of a phenomenon in which the coolant goes on dissolving in the oil that is present in an oil reservoir space provided inside an airtight vessel of the compressor.
Such a phenomenon is called a coolant liquefaction phenomenon. As a result of the coolant liquefaction phenomenon, the liquid coolant dilutes the oil present in the oil reservoir. Moreover, at the time of restarting the compressor; the coolant, which has dissolved in the oil present in the oil reservoir, elutes in the form of bubbles thereby leading to a foaming phenomenon. Or else, if the compression element directly absorbs the liquid coolant, then that may result in liquid compression thereby causing malfunctioning of the compressor.
In a typical air-conditioning apparatus, with the aim of avoiding malfunctioning of the compressor caused by the coolant liquefaction phenomenon; a signal is output when the quantity of the liquid coolant accumulated inside the compressor becomes equal to or greater than a predetermined quantity, and a control unit that receives the output signal passes a high-frequency open-phase current of low intensity to a motor winding so that the motor winding is warmed. With that, a situation is prevented from occurring in which liquid compression is caused due to the start of operation in a state when the liquid coolant inside the compressor is accumulated at a low temperature. As a result, the compressor is prevented from falling into disrepair (for example, see Patent Literature 1).